In a typical radar system, a radar antenna outputs a modulated signal in the direction of an object which is the subject of the radar inquiry. The object reflects the emitted signal back toward the radar system and the returned signal is compared against a saved version of the originally emitted signal. Based on correlations between the emitted and returned signals, various information regarding the target can be determined. For example, in relatively simple systems, the relative velocity between the object and the radar system can be determined in accordance with the Doppler shift between the emitted and returned signals. In more complex radar systems, signal processing techniques performed on the reflected signal yield data regarding the size, shape, range, and direction of the object.
In some radar applications, a radar defeat system, such as a standoff jammer, attempts to defeat the radar system by detecting the emitted signal, copying the emitted signal, and returning a modified version of the copied signal so that the modified signal deceives the radar system. Where the radar signal periodically repeats, the radar defeat system may simply copy and send a delayed version of the copied radar signal to deceive the radar system.
A radar signal having an easily detected and repeatable modulation scheme is more easily copied. For example, a conventional radar system may yield the range, velocity, and bearing to a target. In such applications, standoff jammers store the incoming radar signal in memory and determine the repeat interval of the signal. The standoff jammer then emits a signal replicating the return signal expected by the radar system. The signal emitted by the standoff jammer is delayed so that the return signal emitted by the standoff jammer arrives before the return signal would normally arrive for a given distance between the radar system and the object. This signal deceives the radar system into determining that the object is at a different range, velocity, and bearing than it actually is.
Existing radar systems often use repetitive waveforms to enable range determination. A pseudo-random noise sequence provides one example of a modulation waveform. In the pseudorandom noise sequence, the radar system emits binary sequences characteristic of a noise waveform, but which is repeated after a predetermined timed interval. Another modulation format is frequency modulation continuous wave (FM/CW), also referred to as swept-frequency or chirp waveforms. Yet another modulation format is medium pulse repetition. In medium pulse radar, the transmit waveform modulation is a train of pulses, and the range to an object is determined by the delay between transmission and reception of the pulse.
The above-described modulation waveforms are deterministic. The deterministic characteristic enables intelligent standoff repeater jammers to store the radar signal, modify the signal, and retransmit the delayed signal back toward the radar system so that the target appears other than it actually is. Thus, the radar system receives and interprets a return signal which does not properly indicate the object.
A radar system may also have multiple radio frequency phases. For example, a 255-bit or 511-bit, maximal-sequence, pseudo-random waveform modulation may be used to detect the range from the radar to the target. This code bi-phase modulates the radio frequency (RF) carrier using a binary sequence. The modulation of the signal returned from the target is correlated with delayed images of the originally emitted 255-bit code. A correlation occurs when the delay is equivalent to the target range. Samples of the correlated output are then processed by standard signal processing techniques so that the target is detected.
For example, assume an approximate signal propagation velocity of one foot per nanosecond and a straight line two-way travel path. If the time delay of a single bit of the code modulation is ten nanoseconds (ns) then a delay of one bit in the returned signal would indicate a distance to the target of five feet. Similarly, five bits or 50 nanoseconds of delay indicates a range of 25 feet, and a 255-bit delay indicates a range of 1275 feet. This range for a 255-bit, 1275 feet, is called the unambiguous range. Delays beyond 255-bits fall into an ambiguous range. For example, a delay of 256-bits indicates a range of five feet because the periodic nature of the 255-bit code. Thus, if a standoff radar jammer can store the repetitive waveform, amplify it, and transmit the waveform back with the proper delay, the target carrying the jammer can be made to look closer in range than it actually is.
In addition to radar systems, electronic communication can similarly be disrupted by jammer units. For example, when a communication jammer unit detects a communication signal, the jammer unit can search for repetitive waveforms within the communication signal, duplicate those waveforms, and transmit a modified copy of the signal. This modified signal no longer contains the same information as the originally emitted signal.
Thus, it is an object of the present invention to provide radar and communication systems which emit and receive an electromagnetic signal which is substantially immune from standoff jamming.
It is a further object of the present invention to provide radar and communication systems which send and receive modulated electromagnetic signals having information encoded therein.
It is yet a further object of the present invention to provide radar and communication systems which emit an electromagnetic signal modulated by a local oscillator and receive an electromagnetic signal which may be demodulated by a similar local oscillator.
It is yet a further object of the present invention to provide radar and communication systems which use independent, adaptive modulation controls for local oscillators in the transmitter and receiver to filter out a repeater jammer signal while maintaining a good signal-to-noise ratio (SNR) on the radar target or communication signal.
It is yet a further object of the present invention to provide radar and communication systems having anti-jam capability which adjust the transmit and receive modulation signals to minimize the signal-to-ratio at the jammer range while maximizing the signal-noise-ratio at the radar target range or to the communication system range.
It is yet a further object of the present invention to provide radar and communication systems having anti-jam capability in which the anti-jam modulations are sufficiently flexible to continue tuning to the radar and communication signal while the range to the jammer and to the target or other communication system are both changing.